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Asteroids, sometimes called minor planets or
planetoids, are small Solar System bodies in orbit around the
Sun, especially in the inner Solar System; they
are smaller than planets but larger than meteoroids. The term "asteroid" has historically
been applied primarily to minor planets of the inner Solar System,
as the outer Solar System was
poorly known when it came into common usage. The distinction
between asteroids and comets is made on visual
appearance: Comets show a perceptible coma while asteroids do not.

Terminology

Traditionally, small bodies orbiting the Sun were classified as
asteroids, comets or meteoroids, with anything smaller than ten metres
across being called a meteoroid. The term "asteroid" is somewhat
ill-defined. It never had a formal definition, with the broader
term minor planet being preferred by
the International
Astronomical Union until 2006, when the term "small Solar System body" (SSSB) was
introduced to cover both minor planets and comets. The 2006
definition of SSSB says that they "include most of the Solar System
asteroids, most Trans-Neptunian Objects (TNOs), comets, and other
small bodies". Other languages prefer "planetoid" (Greek for
"planet-like"), and this term is occasionally used in English for
the larger asteroids. The word "planetesimal" has a similar meaning, but refers
specifically to the small building blocks of the planets that
existed at the time the Solar System was forming. The term
"planetule" was coined by the geologist William Daniel Conybeare to
describe minor planets, but is not in common use.

When found, asteroids were seen as a class of objects distinct from
comets, and there was no unified term for the two until "small
Solar System body" was coined in 2006. The main difference between
an asteroid and a comet is that a comet shows a coma due to
sublimation of near surface ices by solar
radiation. A few objects have ended up being dual-listed because
they were first classified as minor planets but later showed
evidence of cometary activity. Conversely, some (perhaps all)
comets are eventually depleted of their surface volatile ices and become asteroids. A further
distinction is that comets typically have more eccentric orbits
than most asteroids; most "asteroids" with notably eccentric orbits
are probably dormant or extinct comets.

For almost two centuries, from the discovery of the first asteroid,
Ceres, in 1801 until the
discovery of the first centaur, 2060
Chiron, in 1977, all known asteroids spent most of their time
at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part
of their orbit. When astronomers started finding additional small
bodies that permanently resided further out than Jupiter, now
called centaurs, they
numbered them among the traditional asteroids, though there was
debate over whether they should be classified as asteroids or as a
new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially
when large numbers of similar objects started turning up, new terms
were invented to sidestep the issue: Kuiper Belt object (KBO), trans-Neptunian object (TNO),
scattered-disc object (SDO),
and so on. These inhabit the cold outer reaches of the Solar System
where ices remain solid and comet-like bodies are not expected to
exhibit much cometary activity; if centaurs or TNOs were to venture
close to the Sun, their volatile ices would sublimate, and
traditional approaches would classify them as comets rather than
asteroids.

The innermost of these are the Kuiper Belt Objects (KBOs), called
"objects" partly to avoid the need to classify them as asteroids or
comets. KBOs are believed to be predominantly comet-like in
composition, though some may be more akin to asteroids.
Furthermore, most do not have the highly eccentric orbits
associated with comets, and the ones so far discovered are very
much larger than traditional comet
nuclei. (The much more distant Oort
cloud is hypothesized to be the main reservoir of dormant
comets.) Other recent observations, such as the analysis of the
cometary dust collected by the Stardust probe, are increasingly
blurring the distinction between comets and asteroids, suggesting
"a continuum between asteroids and comets" rather than a sharp
dividing line.

The minor planets beyond Jupiter's orbit are rarely directly
referred to as "asteroids", but all are commonly lumped together
under the term "asteroid" in popular presentations. For instance, a joint
NASA-JPL public-outreach website states,

It is, however, becoming increasingly common for the term
"asteroid" to be restricted to minor planets of the inner Solar
System, and therefore this article will restrict itself for the
most part to the classical asteroids: objects of the main asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small solar system bodies in 2006 to
include most objects previously classified as minor planets and
comets, they created the class of dwarf
planets for the largest minor planets—those which have
sufficient mass to have become ellipsoidal under their own gravity.
According to the IAU, "the term 'minor planet' may still be used,
but generally the term 'small solar system body' will be
preferred." Currently only the largest object in the asteroid belt,
Ceres, at about across, has
been placed in the dwarf planet category, although there are
several large asteroids (Vesta, Pallas, and Hygiea) that
may be classified as dwarf planets when their shapes are better
known.

Formation

It is believed that planetesimals in
the main asteroid belt evolved much like the rest of the Solar Nebula until Jupiter neared its current
mass, at which point excitation from orbital resonances with Jupiter ejected
over 99% of planetesimals in the belt. Both simulations and a
discontinuity in spin rate and spectral properties suggest that
asteroids larger than approximately in diameter accreted during
that early era, whereas smaller bodies are fragments from
collisions between asteroids during or after the Jovian disruption.
At least two asteroids, Ceres and Vesta, grew large enough to melt
and differentiate, with heavy metallic elements sinking to the
core, leaving rocky minerals in the crust.

In the Nice model, a large number of
Kuiper Belt objects are captured
in the outer Main Belt, at distances greater than 2.6 AU. Most were
subsequently ejected by Jupiter, but those that remained may be the
D-type asteroids, and possibly
include Ceres.

Characteristics

Objects in the main asteroid belt vary greatly in size, from a
diameter of 950 kilometres for the dwarf planet Ceres and over 500 kilometres for the
asteroids 2 Pallas and 4
Vesta down to rocks just tens of metres across. The three
largest are very much like miniature planets: they are roughly
spherical, have at least partially differentiated interiors, and
indeed are thought to be surviving protoplanets. The vast majority, however, are
much smaller and are irregularly shaped; they are thought to be
either surviving planetesimals or
fragments of larger bodies.

The physical composition of asteroids is varied and in most cases
poorly understood. Ceres appears to be composed of a rocky core
covered by an icy mantle, whereas Vesta is thought to have a
nickel-iron core, olivine mantle, and
basaltic crust. 10 Hygiea, on the other
hand, which appears to have a uniformly primitive composition of
carbonaceouschondrite, is thought to be the largest
undifferentiated asteroid. Many, perhaps most, of the smaller
asteroids are piles of rubble held together loosely by gravity.
Some have moons or are co-orbiting
binary asteroids. The rubble piles,
moons, binaries, and scattered asteroid families are believed to be the
results of collisions which disrupted a parent asteroid.

Asteroids contain traces of amino-acids and other organic
compounds, and some speculate that asteroid impacts may have seeded
the early Earth with the chemicals necessary to initiate life, or
may have even brought life itself to Earth. (See also panspermia.)

Only one asteroid, 4 Vesta, which has a particularly reflective
surface, is normally visible to the naked eye, and this only in
very dark skies when it is favorably positioned. Very rarely, small
asteroids passing close to Earth may be naked-eye visible for a
short period of time.

The orbits of asteroids are often influenced by the gravity of
other bodies in the solar system or the Yarkovsky effect.

Distribution within the Solar System

The vast majority of known asteroids orbit within the main asteroid belt between the orbits of
Mars and Jupiter,
generally in relatively low-eccentricity (i.e., not very elongated)
orbits. This belt is currently estimated to contain between 1.1 and
1.9 million asteroids larger than in diameter, and millions of
smaller ones. It is thought that these asteroids are remnants of
the protoplanetary disk, and in
this region the accretion
of planetesimals into planets during
the formative period of the solar system was prevented by large
gravitational perturbations by Jupiter.
Although fewer Trojan asteroids
sharing Jupiter's orbit are currently known, it is thought that
there are as many as there are asteroids in the main belt.

The dwarf planetCeres is the largest object in the
asteroid belt, with a diameter of over . The next largest are the
asteroids 2 Pallas and 4
Vesta, both with diameters of over . Normally Vesta is the only
main belt asteroid that can, on occasion, become visible to the
naked eye. However, on some very rare occasions, a near-Earth
asteroid may briefly become visible without technical aid; see
99942 Apophis.

The mass of all the objects of the Main
asteroid belt, lying between the orbits of Mars and Jupiter, is estimated
to be about 3.0-3.6 kg, or about 4 percent of the mass of the
Moon. Of this, Ceres comprises
0.95 kg, some 32 percent of the total. Adding in the next
three most massive objects, Vesta (9%),
Pallas (7%), and Hygiea (3%), brings this figure up to 51%; while
the three after that, 511 Davida (1.2%),
704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total
mass. The number of asteroids then increases rapidly as their
individual masses decrease.

Various classes of asteroid have been discovered outside the main
asteroid belt. Near-Earth
asteroids have orbits in the vicinity of Earth's orbit.
Trojan asteroids are
gravitationally locked into synchronisation with Jupiter, either leading or trailing the planet in
its orbit. A couple trojans have
been found orbiting with Mars. A group of
asteroids called Vulcanoids are
hypothesised by some to lie very close to the Sun, within the orbit
of Mercury, but none has so far
been found.

Classification

Asteroids are commonly classified according to two criteria: the
characteristics of their orbits, and features of their reflectance
spectrum.

Orbit groups and families

Many asteroids have been placed in groups and families based on
their orbital characteristics. Apart from the broadest divisions,
it is customary to name a group of asteroids after the first member
of that group to be discovered. Groups are relatively loose
dynamical associations, whereas families are much tighter and
result from the catastrophic break-up of a large parent asteroid
sometime in the past. Families have only been recognized within the
main asteroid belt. They were first
recognised by Kiyotsugu Hirayama
in 1918 and are often called Hirayama
families in his honor.

About 30% to 35% of the bodies in the main belt belong to dynamical
families each thought to have a common origin in a past collision
between asteroids. A family has also been associated with the
plutoid dwarf planet .

Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few
hundred years, before returning to their prior status. Both Earth
and Venus are known to have
quasi-satellites.

Such objects, if associated with Earth or Venus or even
hypothetically Mercury, are a
special class of Aten asteroids.
However, such objects could be associated with outer planets as
well.

Spectral classification

In 1975, an asteroid taxonomic system based
on colour, albedo, and
spectral shape was developed by
Clark R.Chapman, David
Morrison, and Ben Zellner. These
properties are thought to correspond to the composition of the
asteroid's surface material. The original classification system had
three categories: C-type for dark
carbonaceous objects (75% of known asteroids), S-type for stony (silicaceous) objects (17%
of known asteroids) and U for those that did not fit into either C
or S. This classification has since been expanded to include a
number of other asteroid types. The number of types continues to
grow as more asteroids are studied.

The two most widely used taxonomies currently used are the Tholen classification and SMASS classification. The former was
proposed in 1984 by David J.Tholen, and was based on data
collected from an eight-color asteroid survey performed in the
1980s. This resulted in 14 asteroid categories. In 2002, the Small
Main-Belt Asteroid Spectroscopic Survey resulted in a modified
version of the Tholen taxonomy with 24 different types. Both
systems have three broad categories of C, S, and X asteroids, where
X consists of mostly metallic asteroids, such as the M-type. There are also a number of smaller
classes.

Note that the proportion of known asteroids falling into the
various spectral types does not necessarily reflect the proportion
of all asteroids that are of that type; some types are easier to
detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an
asteroid's composition. However, the correspondence between
spectral class and composition is not always very good, and there
are a variety of classifications in use. This has led to
significant confusion. While asteroids of different spectral
classifications are likely to be composed of different materials,
there are no assurances that asteroids within the same taxonomic
class are composed of similar materials.

At present, the spectral classification based on several coarse
resolution spectroscopic surveys in the 1990s is still the
standard. Scientists have been unable to agree on a better
taxonomic system, largely due to the difficulty of obtaining
detailed measurements consistently for a large sample of asteroids
(e.g. finer resolution spectra, or non-spectral data such as
densities would be very useful).

Discovery

The first named minor planet, Ceres, was discovered in 1801 by
Giuseppe Piazzi, and was originally
considered a new planet. This was followed by the discovery of
other similar bodies, which with the equipment of the time appeared
to be points of light, like stars, showing little or no planetary
disc (though readily distinguishable from stars due to their
apparent motions). This prompted the astronomer Sir William Herschel to propose the term
"asteroid", from Greek αστεροειδής, asteroeidēs =
star-like, star-shaped, from ancient Greek Aστήρ,
astēr = star. In the early second half of the nineteenth
century, the terms "asteroid" and "planet" (not always qualified as
"minor") were still used interchangeably; for example, the Annual of Scientific Discovery for
1871, page 316, reads "Professor J. Watson has been
awarded by the Paris Academy of Sciences, the astronomical prize,
Lalande foundation, for the discovery of 8 new
asteroids in one year. The planet
Lydia (No. 110), discovered by M. Borelly at the Marseilles
Observatory [...] M. Borelly had previously discovered 2
planets bearing the numbers 91 and 99 in the
system of asteroids revolving between Mars and
Jupiter" (emphasis added).

Historical methods

Asteroid discovery methods have dramatically improved over the past
two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group
of 24 astronomers to search the sky for the missing planet
predicted at about 2.8 AU from the
Sun by the Titius-Bode law, partly
as a consequence of the discovery, by Sir William Herschel in 1781, of the planet
Uranus at the distance predicted by the law.
This task required that hand-drawn sky charts be prepared for all
stars in the zodiacal band down to an
agreed-upon limit of faintness. On subsequent nights, the sky would
be charted again and any moving object would, hopefully, be
spotted. The expected motion of the missing planet was about 30
seconds of arc per hour, readily discernible by observers.

The first
asteroid, 1 Ceres, was not
discovered by a member of the group, but rather by accident in 1801
by Giuseppe Piazzi, director of the
observatory of Palermo in Sicily. He discovered a new star-like object
in Taurus and followed the
displacement of this object during several nights. His colleague,
Carl Friedrich Gauss, used
these observations to determine the exact distance from this
unknown object to the Earth. Gauss' calculations placed the object
between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of
agriculture.

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were
discovered over the next few years, with Vesta found in 1807. After
eight more years of fruitless searches, most astronomers assumed
that there were no more and abandoned any further searches.

In 1891, however, Max Wolf pioneered
the use of astrophotography to
detect asteroids, which appeared as short streaks on long-exposure
photographic plates. This dramatically increased the rate of
detection compared with previous visual methods: Wolf alone
discovered 248 asteroids, beginning with 323
Brucia, whereas only slightly more than 300 had been discovered
up to that point. Still, a century later, only a few thousand
asteroids were identified, numbered and named. It was known that
there were many more, but most astronomers did not bother with
them, calling them "vermin of the skies".

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process.
First, a region of the sky was photographed by a wide-field telescope, or Astrograph. Pairs of photographs were taken,
typically one hour apart. Multiple pairs could be taken over a
series of days. Second, the two films of the
same region were viewed under a stereoscope. Any body in orbit around the Sun
would move slightly between the pair of films. Under the
stereoscope, the image of the body would appear to float slightly
above the background of stars. Third, once a moving body was
identified, its location would be measured precisely using a
digitizing microscope. The location would be measured relative to
known star locations.

These first three steps do not constitute asteroid discovery: the
observer has only found an apparition, which gets a provisional
designation, made up of the year of discovery, a letter
representing the half-month of discovery, and finally a letter and
a number indicating the discovery's sequential number (example:
).

The final step of discovery is to send the locations and time of
observations to the Minor Planet
Center, where computer programs determine whether an apparition
ties together previous apparitions into a single orbit. If so, the
object receives a catalogue number and the observer of the first
apparition with a calculated orbit is declared the discoverer, and
granted the honor of naming the object subject to the approval of
the International
Astronomical Union.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898,
and the 1930s brought a flurry of similar objects. In order of
discovery, these were: 1221 Amor, 1862 Apollo, 2101
Adonis, and finally 69230 Hermes,
which approached within 0.005 AU
of the Earth in 1937. Astronomers began to
realize the possibilities of Earth impact.

Two events in later decades increased the level of alarm: the
increasing acceptance of Walter
Alvarez' hypothesis that an impact
event resulted in the Cretaceous-Tertiary
extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into
Jupiter. The U.S. military also declassified
the information that its military satellites, built to detect
nuclear explosions, had detected hundreds of upper-atmosphere
impacts by objects ranging from one to 10 metres across.

All of these considerations helped spur the launch of highly
efficient automated systems that consist of Charge-Coupled Device
(CCD) cameras and computers
directly connected to telescopes. Since 1998, a large majority of
the asteroids have been discovered by such automated systems. A
list of teams using such automated systems includes:

The LINEAR system alone has discovered 97,470 asteroids, as of
September 18, 2008. Between all of the automated systems, 4711
near-Earth asteroids have been discovered including over 600 more
than in diameter. The rate of discovery peaked in 2000, when 38,679
minor planets were numbered, and has been going down steadily since
then (719 minor planets were numbered in 2007).

Naming

A newly discovered asteroid is given a provisional designation
(such as ) consisting of the year of discovery and an alphanumeric
code indicating the half-month of discovery and the sequence within
that half-month. Once an asteroid's orbit has been confirmed, it is
given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses
parentheses around the number (e.g. (433) Eros), but dropping the
parentheses is quite common. Informally, it is common to drop the
number altogether, or to drop it after the first mention when a
name is repeated in running text.

Symbols

The first few asteroids discovered were assigned symbols like the
ones traditionally used to designate Earth, the Moon, the Sun and
planets. The symbols quickly became ungainly, hard to draw and
recognize. By the end of 1851 there were 15 known asteroids, each
(except one) with its own symbol(s).

Johann Franz Encke made a major
change in the Berliner Astronomisches Jahrbuch (BAJ, Berlin
Astronomical Yearbook) for 1854. He introduced encircled numbers
instead of symbols, although his numbering began with Astraea, the first four asteroids continuing to be
denoted by their traditional symbols. This symbolic innovation was
adopted very quickly by the astronomical community. The following
year (1855), Astraea's number was bumped up to 5, but Ceres through
Vesta would be listed by their numbers only in the 1867 edition. A
few more asteroids (28 Bellona, 35 Leukothea, and 37
Fides) would be given symbols as well as using the numbering
scheme. The circle would become a pair of parentheses, and the
parentheses sometimes omitted altogether over the next few
decades.

Exploration

Vesta, imaged by the Hubble Space
Telescope

Until the age of space travel, objects
in the asteroid belt were merely pinpricks of light in even the
largest telescopes and their shapes and terrain remained a mystery.
The best modern ground-based telescopes, as well as the
Earth-orbiting Hubble Space
Telescope, can resolve a small amount of detail on the surfaces
of the very largest asteroids, but even these mostly remain little
more than fuzzy blobs. Limited information about the shapes and
compositions of asteroids can be inferred from their light curves (their variation in brightness as
they rotate) and their spectral properties, and asteroid sizes can
be estimated by timing the lengths of star occulations (when an
asteroid passes directly in front of a star). Radar imaging can yield good information about
asteroid shapes and orbital and rotational parameters, especially
for near-Earth asteroids.

The first close-up photographs of asteroid-like objects were taken
in 1971 when the Mariner 9 probe imaged
Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These
images revealed the irregular, potato-like shapes of most
asteroids, as did subsequent images from the Voyager probes of the small moons of the
gas giants.

In September 2005, the Japanese Hayabusa
probe started studying 25143 Itokawa
in detail and may return samples of its surface to earth. The
Hayabusa mission has been plagued with difficulties, including the
failure of two of its three control wheels, rendering it difficult
to maintain its orientation to the sun to collect solar energy.
Following that, the next asteroid encounters will involve the
European Rosetta probe (launched
in 2004), which flew by 2867 Šteins
in 2008 and will buzz 21 Lutetia in
2010.

In
September 2007, NASA launched the
Dawn Mission, which will orbit the
dwarf planet Ceres and the
asteroid 4 Vesta in 2011-2015, with its
mission possibly then extended to 2
Pallas.

In fiction

Asteroids and asteroid belts are a staple of science fiction
stories. Asteroids play several potential roles in science fiction:
as places which human beings might colonize; as resources for
extracting minerals; as a hazard encountered by spaceships
travelling between two other points; and as a threat to life on
Earth due to potential impacts.

Neptune also has a few known trojans, and these are thought to
be actually be much more numerous than the Jovian trojans. However,
they are often included in the trans-Neptunian population rather
than counted with the asteroids.

Ceres, originally considered a new planet, is the largest main
belt object and is now classified as a dwarf planet. All other asteroids are now
classified as small solar system bodies along with
comets, centaurs, and the smaller TNOs.